Abrasive wheels which are both very thin and highly stiff are commercially important. For example, thin abrasive wheels are used in cutting off thin sections and in performing other abrading operations in the processing of silicon wafers and so-called pucks of alumina-titanium carbide composite in the manufacture of electronic products. Silicon wafers are generally used for integrated circuits and alumina-titanium carbide pucks are utilized to fabricate flying thin film heads for recording and playing back magnetically stored information. The use of thin abrasive wheels to abrade silicon wafers and alumina-titanium carbide pucks is explained well in U.S. Pat. No. 5,313,742, the entire disclosure of which patent is incorporated herein by reference. As stated in the '742 patent, the fabrication of silicon wafers and alumina-titanium carbide pucks creates the need for dimensionally accurate cuts with little waste of the work piece material. Ideally, cutting blades to effect such cuts should be as stiff as possible and as thin and flat as practical because the thinner and flatter the blade, the less kerf waste produced and the stiffer the blade, the more straight it will cut. However, these characteristics are in conflict because the thinner the blade, the less rigid it becomes.
Cutting blades are made up basically of abrasive grains and a bond which holds the abrasive grains in the desired shape. Because bond hardness tends to increase with increased stiffness, it would seem logical to raise bond hardness to obtain a stiffer blade. However, a hard bond also has more wear resistance which can retard bond erosion so that the grains become dull before being expelled from the blade. Despite being very stiff, a hard bonded blade demands aggressive dressing and so is less desirable.
Industry has evolved to using monolithic abrasive wheels, usually ganged together on an arbor. Individual wheels in the gang are axially separated from each other by incompressible and durable spacers. Traditionally, the individual wheels have a uniform axial dimension from the wheel's arbor hole to its periphery. Although quite thin, the axial dimension of these wheels is greater than desired to provide adequate stiffness for good accuracy of cut. However, to keep waste generation within acceptable bounds, the thickness is reduced. This diminishes rigidity of the wheel to less than the ideal.
The conventional straight wheel is thus seen to generate more work piece waste than a thinner wheel and to produce more chips and inaccurate cuts than would a stiffer wheel. The '742 patent sought to improve upon performance of ganged straight wheels by increasing the thickness of an inner portion extending radially outward from the arbor hole. The patent discloses that a monolithic wheel with a thick inner portion was stiffer than a straight wheel with spacers. However, the '742 patent wheel suffers from the drawback that the inner portion is not used for cutting, and therefore, the volume of abrasive in the inner portion is wasted. Because thin abrasive wheels, especially those for cutting alumina-titanium carbide, employ expensive abrasive substances such as diamond, the cost of a '742 patent wheel is high compared to a straight wheel due to the wasted abrasive volume.
Heretofore, a metal bond normally has been used for straight, monolithic, thin abrasive wheels intended for cutting hard materials such as silicon wafers and alumina-titanium carbide pucks. A variety of metal bond compositions for holding diamond grains, such as copper, zinc, silver, nickel, or iron alloys, for example, are known in the art. U.S. Pat. No. 3,886,925 discloses a wheel with an abrasive layer formed of high purity nickel electrolytically deposited from nickel solutions having finely divided abrasive suspended in them. U.S. Pat. No. 4,180,048 discloses an improvement to the wheel of the '925 patent in which a very thin layer of chromium is electrolytically deposited on the nickel matrix. U.S. Pat. No. 4,219,004 discloses a blade comprising diamond particles in a nickel matrix which constitutes the sole support of the diamond particles.
A new, very stiff metal bond suitable for binding diamond grains in a thin abrasive wheel has now been discovered. The novel bond composition of nickel and tin with a stiffness enhancing metal component, preferably tungsten, molybdenum, rhenium or a mixture of them provides a superior combination of stiffness, strength and wear resistance. By maintaining the stiffness enhancer within proper proportion to nickel and tin, one can obtain the desired bond properties by pressureless sintering or hot pressing. Thus, while using conventional powder metallurgy equipment, the novel bond can readily supplant traditional, less stiff, bronze alloy based bonds and electroplated nickel bonds.
Accordingly, there is provided an abrasive wheel comprising an abrasive disk consisting essentially of about 2.5-50 vol. % abrasive grains and a complemental amount of a sintered bond of a composition comprising a metal component consisting essentially of nickel and tin, and a stiffness enhancing metal selected from the group consisting of molybdenum, rhenium, tungsten and a mixture of them.
There is also provided a method of cutting a work piece comprising the step of contacting the work piece with at least one abrasive wheel comprising an abrasive disk consisting essentially of about 2.5-50 vol. % abrasive grains and a complemental amount of a sintered bond of a composition comprising a metal component consisting essentially of nickel and tin, and a stiffness enhancing metal selected from the group consisting of molybdenum, rhenium, tungsten and a mixture at least two of them
Still further this invention provides a method of making an abrasive tool comprising the steps of
(a) providing preselected amounts of particulate ingredients comprising PA1 (b) mixing the particulate ingredients to form a uniform composition; PA1 (c) placing the uniform composition into a mold of preselected shape; PA1 (d) compressing the mold to a pressure in the range of about 345-690 MPa for a duration effective to form a molded article; PA1 (e) heating the molded article to a temperature in the range of about 1050-1200.degree. C. for a duration effective to sinter the bond composition; and PA1 (f) cooling the molded article to form the abrasive tool.
(1) abrasive grains; and PA2 (2) a bond composition consisting essentially of nickel powder, tin powder and a stiffness enhancing metal powder selected from the group consisting of molybdenum, rhenium, tungsten and a mixture of them;
Additionally, there is now provided a composition for a sintered bond of a monolithic abrasive wheel comprising a metal component consisting essentially of nickel and tin, and a stiffness enhancing metal selected from the group consisting of molybdenum, rhenium, tungsten and a mixture of at least two of them in which the sintered bond has an elastic modulus of at least about 130 GPa and a Rockwell B hardness less than about 105.